1. Field of the Invention
This invention relates to an improved process for the production of acrylonitrile by the catalytic ammoxidation of propylene in a fluidized-bed reactor, which can produce acrylonitrile in a high yield at a low production cost.
2. Background Art
Reaction for synthesizing acrylonitrile by means of the ammoxidation of propylene is, in general, accompanied by high heat generation. For this reason, a method using a fluidized-bed reactor, which is advantageous to the removal of heat generated, is often employed. However, the conventional technique using such a method tends to have the following problems: the back mixing of a gas is unavoidable due to the violent mixing of particles which is caused in a fluidized bed; and the yield of acrylonitrile is lowered due to the progress of unfavorable side reaction which is caused when contact between a gas and a catalyst becomes insufficient because of bubbles formed in a fluidized-bed reaction area. In particular, when it is intended to attain high productivity by using a highly-active catalyst, not only the yield of acrylonitrile is remarkably lowered, but also it becomes difficult to remove heat of reaction. As a result, it becomes impossible to maintain the reaction temperature in a desirable range, and finally becomes impossible to continue the reaction. In addition, in the case of a highly-active catalyst, it is enough to use it in only a small amount, so that the volume of the catalyst bed is small. The optimum height of the catalyst bed is therefore short, so that it may be difficult to secure, in the catalyst bed, the heat transfer surface area on a cooling coil needed for the removal of heat of reaction and that the influence of back mixing may become great.
As solutions for the aforementioned problems, there have been known, for example, those methods which are proposed in Japanese Patent Publications No. 28491/1969, No. 531/1973 and No. 38428/1983, and U.S. Pat. No. 3,230,246 and No. 3,783,528. In these methods, obstacles such as shaped metallic articles, screens, grids, perforated plates, horizontal plates, pipes or the like are laid in a catalyst bed to prevent the coalescence or growth of bubbles, or to prevent the back mixing of a gas, thereby improving contact between the feed gas and the catalyst. However, as far as the present inventors know, these methods are not practical because construction for laying the obstacles is complicated; and, in addition, they seem to have the following problems: the mixing of catalyst particles is prevented by the obstacles, so that the favorable features of fluidized bed (e.g., excellent temperature controllability, and uniform catalyst bed temperature) are lost; and the distribution of the catalyst in the reactor becomes uneven in terms of space and time, so that it becomes difficult to stably and continuously conduct the operation.
In order to prevent the lowering of contact efficiency that is caused by the back mixing of a gas and by the formation of bubbles, Japanese Patent Laid-Open Publication No. 144528/1978 proposes a method for attaining high acrylonitrile yield by using a fluidized bed with a high gas velocity of approximately 1.5 to 7.5 m/sec. However, even this method seems to be unsatisfactory for economically attaining high productivity by the use of a highly-active catalyst. A particular problem in this method is that a considerably long reactor is required to attain a predetermined rate of reaction because the superficial gas velocity in a column is high. An increased cost is needed to construct such a reactor for use in an industrial-scale apparatus, and this becomes a great economical burden. To avoid this problem, it may be contemplated to make the reactor in the shape of a coil. In this case, however, another problem is brought about as follows: a gas and a catalyst are to flow in the coil with a circling movement, so that they are separated from each other due to centrifugal force, and the efficiency of contact between them is thus lowered. This problem is a fatal drawback in large-scale apparatuses for industrial use.